chore(06-02): download and include Action Scheduler library

- Downloaded Action Scheduler 3.9.3 from GitHub
- Placed in vendor/action-scheduler/ directory
- Included in main plugin file before other code for proper initialization
- Library will auto-initialize itself when required

vendor/action-scheduler/classes/migration/DryRun_LogMigrator.php

@@ -0,0 +1,23 @@
+<?php
+
+
+namespace Action_Scheduler\Migration;
+
+/**
+ * Class DryRun_LogMigrator
+ *
+ * @package Action_Scheduler\Migration
+ *
+ * @codeCoverageIgnore
+ */
+class DryRun_LogMigrator extends LogMigrator {
+	/**
+	 * Simulate migrating an action log.
+	 *
+	 * @param int $source_action_id Source logger object.
+	 * @param int $destination_action_id Destination logger object.
+	 */
+	public function migrate( $source_action_id, $destination_action_id ) {
+		// no-op.
+	}
+}